title: Variable circular collimator in robotic radiosurgery: a time-efficient alternative to a mini-multileaf collimator?
abstract: OBJECTIVE
  Compared with many small circular beams used in CyberKnife treatments, beam's eye view-shaped fields are generally more time-efficient for dose delivery. However, beam's eye view-shaping devices, such as a mini-multileaf collimator (mMLC), are not presently available for CyberKnife, although a variable-aperture collimator (Iris, 12 field diameters; 5-60 mm) is available. We investigated whether the Iris can mimic noncoplanar mMLC treatments using a limited set of principal beam orientations (nodes) to produce time-efficient treatment plans.
  
  
  METHODS
  The data from 10 lung cancer patients and the beam-orientation optimization algorithm "Cycle" were used to generate stereotactic treatment plans (3 × 20 Gy) for a CyberKnife virtually equipped with a mMLC. Typically, 10-16 favorable beam orientations were selected from 117 available robot node positions using beam's eye view-shaped fields with uniform fluence. Second, intensity-modulated Iris plans were generated by inverse optimization of nonisocentric circular candidate beams targeted from the same nodes selected in the mMLC plans. The plans were evaluated using the mean lung dose, lung volume receiving ≥20 Gy, conformality index, number of nodes, beams, and monitor units, and estimated treatment time.
  
  
  RESULTS
  The mMLC plans contained an average of 12 nodes and 11,690 monitor units. For a comparable mean lung dose, the Iris plans contained 12 nodes, 64 beams, and 21,990 monitor units. The estimated fraction duration was 12.2 min (range, 10.8-13.5) for the mMLC plans and 18.4 min (range, 12.9-28.5) for the Iris plans. In contrast to the mMLC plans, the treatment time for the Iris plans increased with an increasing target volume. The Iris plans were, on average, 40% longer than the corresponding mMLC plans for small targets (<80 cm(3)) and ≤121% longer for larger targets. For a comparable conformality index, similar results were obtained.
  
  
  CONCLUSIONS
  For stereotactic lung irradiation, time-efficient and high-quality plans were obtained for robotic-controlled noncoplanar treatments using a mMLC. Iris is a time-efficient alternative for small targets, with similar or better plan quality.
authors.affiliation: Department of Radiation Oncology, Erasmus Medical Center-Daniel den Hoed Cancer Center, Rotterdam, The Netherlands. s.vandewater@erasmusmc.nl
authors.forename: Steven
authors.initials: S
authors.lastname: van de Water
authors.forename: Mischa S
authors.initials: MS
authors.lastname: Hoogeman
authors.forename: Sebastiaan
authors.initials: S
authors.lastname: Breedveld
authors.forename: Joost J M E
authors.initials: JJ
authors.lastname: Nuyttens
authors.forename: Dennis R
authors.initials: DR
authors.lastname: Schaart
authors.forename: Ben J M
authors.initials: BJ
authors.lastname: Heijmen
mesh_terms: D000465:Algorithms; D004867:Equipment Design; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000601:surgery*; D008875:Middle Aged; D058958:Organs at Risk / Q000000981:diagnostic imaging; D011859:Radiography; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods* / Q000592:standards; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods* / Q000592:standards; D061089:Radiotherapy, Image-Guided / Q000379:methods; D012371:Robotics / Q000295:instrumentation / Q000379:methods* / Q000592:standards; D025321:Surgery, Computer-Assisted / Q000295:instrumentation / Q000379:methods / Q000592:standards; D013997:Time Factors